Impact absorbing surfaces (IAS) for playgrounds are now used in preference to concrete as they can reduce the chance of serious injury or death of a child striking them.
A number of structures for these IAS are known, for example, layers of aggregate, typically Lytag™, and/or sand enclosed in a geotextile envelope and topped by a synthetic grass carpet layer. The layers of sand and aggregate are segregated by walls of the envelope in order to prevent depletion of regions of the structure due, for example, to repeated compression in regions subjected to much wear and/or impact such as under a swing, or due to the action of ground water or rain moving the aggregate and/or sand, or to protect the specialist aggregates from migration of ‘foreign’ materials from the sub-structure causing ‘contamination’ of the performance layers. Such compression, movement or contamination of the aggregate and/or sand degrades the performance of the IAS.
These structures have inherent practical and/or logistical problems associated with them such as the need to transport mineral aggregate infill to an installation site. Additionally, spillage of aggregate infill at an installation site is costly as spilled aggregate infill must be removed from the playing surfaces. Further to which in order to achieve a consistent surface layer it is necessary to have level aggregate infill and geotextile envelope structures and this requires labour intensive hand finishing.
Another IAS structure utilises a rubber granulate material which is screeded into and stabilised by a random pile layer which is usually overlain by a resin impregnated textile material. A synthetic grass carpet layer tops the textile material.
Attempts have been made to remove the need for aggregate infill by fabricating a playing surface underlay from multiple layers of a random pile material, for example a material known as vertical horizontal angular fibre (VHAF™) but this has limited applications.
Also, the use of bound rubber tiles or wetpour rubber is known. However, such systems can suffer from breakdown of resins used in the binding of the rubber over time and their performance can degrade accordingly.
Each playing surface must fulfill a standard, the head injury criteria (HIC), which is the integral of the force, measured in G's, applied by a test piece, dropped from a known fall height (measured in metres) onto the playing surface, with respect to time (seconds), i.e. ∫F.dt. The value of the HIC must not exceed 1000 at a given fall height if a playing surface is to be considered appropriate for use at that fall height. A measure of the critical fall height (CFH) is the height at which the HIC reaches a value of 1000. The height at which the maximum force exerted exceeds 200 G can also be taken as a measure of the CFH.
A playing surface constructed from multiple layers of VHAF™ matting cannot achieve high enough CFH values without a substantial number layers being used, more than is economically viable.